1. Field of the Invention
The present invention relates to an improved process for the preparation of aliphatic oligocarbonate diols from non-vicinal diols and dimethyl carbonate (DMC) in a gas-liquid countercurrent apparatus.
2. Description of the Prior Art
Aliphatic oligocarbonates are known as important intermediates, for example, for the manufacture of plastics, lacquers and adhesives, e.g., by reaction with isocyanates. They can be prepared from non-vicinal diols by reaction with phosgene (DE-A 1,595,446), bischlorocarbonic acid esters (DE-A 857,948), diaryl carbonates (DE-A 1,915,908), dioxolanones (DE-A 2,523,352) or dialkyl carbonates (DE-A 2,555,805).
Of the above-mentioned carbonate sources, diphenyl carbonate (DPC) is of particular importance because it can be used to produce aliphatic oligocarbonate diols of particularly high quality (e.g. U.S. Pat. No. 3,544,524, EP-A 292,772).
In contrast to all other carbonate sources, DPC reacts quantitatively with aliphatic OH groups; thus, after the phenol formed has been removed, all the terminal OH groups of the oligocarbonate mixture are available for reaction with isocyanate groups. Also, because only very small concentrations of soluble catalyst are required, it can remain in the product.
Numerous patent applications (e.g. U.S. Pat. No. 2,210,817, U.S. Pat. No. 2,787,632, U.S. Pat. No. 4,169,853, EP-A 364,052) describe the reaction of dialkyl carbonates with aliphatic diols.
The state of the art is to take aliphatic diols together with the catalyst and the dialkyl carbonate and to distil the alcohol formed (ethanol, butanol, allyl alcohol) out of the reaction vessel via a column in such a way that co-vaporized carbonate cannot escape. The reaction is ultimately completed by applying a vacuum to remove unreacted carbonate and residual alcohol. The mixtures are normally only heated and stirred for this purpose.
Despite its good availability, the use of dimethyl carbonate for the preparation of aliphatic oligocarbonate diols has only been known for a short time (U.S. Pat. No. 5,171,830, EP-A 798,327, EP-A 798,328). This can be explained by the low boiling point of DMC and the existence of a low boiling azeotropic mixture with methanol, both of which are an obstacle to the complete conversion of DMC to oligocarbonate diols.
Therefore, these references do not describe processes with high space-time yields and an almost complete reaction of DMC with aliphatic diols to give oligocarbonate diols. The unavoidable occurrence of DMC/methanol mixtures of variable composition appreciably reduces the economic attractiveness of the processes described.
EP-A 798,327 also describes a two-stage process in which initially, with an excess of DMC, an oligocarbonate is produced whose terminal OH groups are in the form of methoxycarbonate groups. Only in a further step, after the addition of another diol and after a total reaction time of 36 hours, is the oligocarbonate diol obtained.
None of these publications considers the practicality of the methods on an industrial scale.
DE-A 19,623,508 describes the transesterification, accelerated by soluble catalysts, of DMC with higher-boiling aliphatic alcohols having only one OH group (monools) in a gas-liquid countercurrent column. To achieve high conversions of DMC (89-99%), it was necessary to accept relatively low conversions of high-boiling alcohol (35 to 58%), suggesting that this technique is unsuitable for the synthesis of high molecular weight oligoesters.
It is an object of the present invention to provide a process for the preparation of oligocarbonate diols by reacting DMC with aliphatic diols, which can be carried out industrially in large, simple apparatus with good space-time yields and a high degree of conversion.
This object may be achieved with the process according to the present invention by carrying out the reaction of dimethyl carbonate and aliphatic diols in the presence of soluble catalysts in a gas-liquid countercurrent apparatus, followed by the separation of residual methanol and traces of dimethyl carbonate in an apparatus which generates gas bubbles in the oligocarbonate.